JPH07116585B2 - Gas nitriding method for thin sheet made of maraging steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for gas nitriding a maraging steel sheet. INDUSTRIAL APPLICABILITY The present invention can be used, for example, in manufacturing a hoop of a steel belt for CVT. Where CVT is C
It is an abbreviation for ontinuously variable transmission, and is formed by a pulley with a variable groove width structure and an endless steel belt installed between the pulleys, and changes the groove width of the pulley to continuously change the gear ratio. It is a continuously variable transmission that adjusts. This CVT steel belt includes an endless belt-shaped hoop and an appropriate number of torque transmission plates held by the hoop. Therefore, the hoop of the CVT steel belt refers to an endless or endless thin plate material used for holding a torque transmission plate that transmits torque in the transmission.

[Prior Art] Maraging steel has an extremely high tensile strength even if it is a thin plate, but it has low wear resistance and low fatigue strength. Therefore, it is used in places where high bending stress is applied. Has a nitriding treatment on a maraging steel thin plate to increase the hardness of the surface portion.

By the way, in performing the nitriding treatment, a gas nitriding method of heating a maraging steel thin plate in an atmosphere of ammonia gas has been conventionally used. However, the thin plate made of maraging steel is difficult to be gas-nitrided due to its high nickel content, and therefore the gas nitriding method has a problem that the treatment time becomes long as 10 to 20 hours. There is a problem that the processing time becomes as long as 24 hours. Although it has been proposed to use a tufftride treatment method, the treatment temperature is as high as 550 ° C. or higher, which is not preferable in order to prevent overaging and deformation of the maraging steel sheet.

Further, a gas soft nitriding method has been proposed in which RX gas is mixed with ammonia gas in order to perform gas nitriding at a low temperature, but this method is not preferable because it tends to increase the carbon content of the thin steel plate made of maraging steel .

[Problems to be Solved by the Invention] The present invention has been made in view of the above situation, and an object thereof is to promote nitriding, to lower the treatment temperature, and
It is another object of the present invention to provide a method for gas nitriding a thin plate made of maraging steel that can shorten the processing time.

[Means for Solving the Problems] As a result of diligent research on a gas nitriding method for a maraging steel thin plate, the present inventor has made ammonia having a smaller decomposition rate of ammonia gas and a longer gas retention time in a retort. If the nitriding treatment of the maraging steel thin plate is performed in a gas atmosphere, the treatment temperature can be lowered.
It can be as low as 420-470 ℃, and the treatment time is 1-
I found that it could be shortened to 6 hours. The reason why the treatment temperature can be lowered and the treatment time can be shortened is not necessarily clear, but if the nitriding treatment is performed in a state where the decomposition rate of ammonia gas is small, the ammonia gas is activated immediately after decomposition. It is presumed that this is because nitrogen can be effectively used. The present invention is based on this finding.

That is, the gas nitriding method of the maraging steel thin plate according to the present invention, charging the maraging steel thin plate into the nitriding furnace, the residence time is 600 ~ 3600 seconds, and the decomposition rate 1 ~
Processing time 1-6 in 6% ammonia gas atmosphere
It is characterized by performing gas nitriding as time.

Here, the roughing of the maraging steel thin plate is appropriately set according to the use of the thin plate, but in% by weight, nickel 17 to 26%,
Aluminum can be 0.05 to 0.5%, molybdenum 4 to 6%, titanium 0.2 to 1.6%, cobalt 7 to 12%, and the balance iron. The thickness of the maraging steel thin plate is appropriately set according to the use of the thin plate, but when applied to the hoop of a CVT steel belt, it is 0.1 to 1.0 mm, especially 0.1 to 0.2 mm.
Can be Solution treatment may be performed before the nitriding treatment of the present invention, the solution treatment temperature and solution treatment time, nickel, aluminum, the temperature at which the dissolved atoms of titanium and the like are sufficiently dissolved in austenite, usually 800 ~ 8
It may be set at 50 ° C for 30 minutes to 5 hours. The aging treatment can be carried out at the same time as the nitriding treatment of the present invention, and only when the treatment temperature is lower than 450 ° C, the nitriding treatment is performed at 450 to 500 ° C for about 30 minutes to 1 hour.

In the nitriding treatment, a maraging steel thin plate is charged into a nitriding treatment furnace, and the treatment temperature is lower than a normal nitriding treatment temperature, for example, 420 to 470 ° C in an ammonia gas atmosphere where the decomposition rate is 1 to 6%. Then, the gas nitriding is performed for 1 to 6 hours. To create an atmosphere of ammonia gas having a decomposition rate of 1 to 6%, for example, the retention time of the ammonia gas in the nitriding furnace is increased, and the volume of the nitriding furnace is increased to increase the ammonia in the nitriding furnace. It may be possible to change the gas late. Therefore, the residence time of ammonia gas is 600 to 3600 seconds. Here, when the surface area of the maraging steel sheet is about 123 cm ² , the internal volume of the nitriding furnace charged with ammonia gas is 30
It is preferable that the retention time is set to 100 and the residence time is set according to the flow rate of the supply gas. The back pressure of the nitriding furnace is a gauge pressure.
It can be 0.001 to 0.03 kg / cm ² . When the nitriding treatment is performed as described above, the surface portion of the maraging steel thin plate is nitrided and a nitride layer is formed on the surface portion. The nitride layer generally has a thickness of about 15 to 30 μ, and therefore the surface hardness of the member is about Hv 800 to 1000. Therefore, the compressive residual stress on the surface of the member is 100 to 150 kg / mm ² .

[Effect of the Invention] In the present invention, the treatment temperature required for nitriding can be lower than the normal nitriding treatment temperature, for example, 420 to 470 ° C, and the treatment time required for nitriding can be set to 1 to 6 hours. Although the reason is not clear, it is presumed that if the nitriding treatment is performed in a state where the decomposition rate of the ammonia gas is small, the activated nitrogen can be effectively used in the early stage of generation, so that the nitriding is promoted. Thus, in the present invention, the processing temperature can be low and the processing time can be shortened, so that the maraging steel thin plate that is easily deformed by heat treatment, for example, the wall thickness of 0.1 to 0.2 mm
Suitable for gas nitriding of thin plates.

[Embodiment] A first embodiment of the present invention is shown in FIG.

As the first step, the thin plate is placed in a vacuum furnace and heated at 800 ° C for 30
By heating for a minute, the solution was formed, and the dissolved atoms such as nickel and titanium were solid-dissolved in austenite. After that, the thin plate was cooled. Cooling was performed with nitrogen gas and argon gas.

Here, the shape of the thin plate is an endless hoop shape, and the size thereof is 0.17 mm in thickness, 8.6 mm in width, and 704 mm in inner peripheral length. The maraging steel sheet is made up by weight%, nickel 17.8%, aluminum 0.08%, molybdenum 4.76%, titanium 0.48%, cobalt 7.75%, carbon 0.005%, silicon 0.038%, sulfur 0.00.
03%, manganese 0.008%, balance iron.

Then, the thin plate 2 is placed on the table 3 in the retort 1, and in that state, the lid 22 is attached to the retort 1 with the bolt 21 via the packing material 20. Here, the retort 1 is made of SUS304, and the internal dimensions are length 350mm, width 350mm, height 300m.
m. The height of the table 3 is 20 mm. A supply pipe 4 for supplying ammonia gas and a discharge pipe 6 for discharging ammonia gas are attached to the retort 1. The tip of the supply pipe 4 is located above the table 3.
Here, ammonia gas is continuously supplied into the retort 1 at a pressure of 1 kg / cm ² and a flow rate of 1.5 / min by a pressure gauge and a flow meter (not shown). At this time, the retention time of ammonia gas in the retort is about 24.5 minutes.

The above retort was housed in a nitriding furnace, and ammonia gas was continuously supplied into the retort 1 from the supply pipe 4 to replace the air in the retort 1 with the ammonia gas. After this, while continuously supplying ammonia gas, the temperature of the nitriding furnace is raised to heat and hold the thin plate 2 at a processing temperature of 440 ° C. for a processing time of 3 hours, whereby the ammonia gas in the retort 1 is decomposed, Nitriding treatment was performed as two steps. In this example, the undecomposed ammonia gas was 96%, that is, the gas decomposition rate was 4%. The back pressure of the retort 1 is 0.003 kg / cm ² as a gauge pressure.

On the surface portion of the thin plate 2 that has been subjected to the nitriding treatment as described above,
A nitride layer was formed. As shown in the table, the thickness of the nitride layer is 20 μm, the surface hardness of the thin plate 2 is HV900, and the compressive residual stress is 1
There was 06 kg / mm ² . The thickness of the nitride layer was calculated from the EPMA concentration distribution. The surface hardness was determined by micro-Vickers with a load of 50 g. The compressive residual stress is X by the X-ray tilt method.
It was determined by measuring the stress with a line.

Next, as a second embodiment, the processing time in the second step is 6
The first step and the second step were performed under the condition that the time was set and the remaining conditions were the same as in the eleventh example. In the thin plate according to the second example, the nitride layer was as thick as 85 μm, the surface hardness of the thin plate was Hv1000, and the compressive residual stress was 152 kg / mm ² . In this example, the nitriding layer was thick as a hoop material and the conditions were not appropriate, but it can be seen that nitriding is performed quickly. As an appropriate condition, the processing temperature may be lowered.

As a comparative example, a thin plate was nitrided in a state where the treatment temperature was 450 ° C., the treatment time was 24 hours, and the gas decomposition rate was 12% (the conventional decomposition rate was an appropriate value). At this time, the retention time of ammonia gas was about 60 seconds. In this comparative example, the thickness of the nitride layer is 25 μm, the surface hardness of the member is Hv850, and the compression residue remains. The stress was 80 kg / mm ² . As is clear from the above test results, the treatment temperature was 440 ° C. and the treatment time was 40% by setting the gas decomposition rate to 4% and prolonging the retention time of ammonia gas as in the first and second examples. It can be seen that a nitride layer having a hardness of Hv900 to 1000 can be formed in about 3 to 6 hours.

[Brief description of drawings]

FIG. 1 is a vertical sectional side view showing a state in which a first embodiment according to the present invention is carried out. In the figure, 1 is a retort, 2 is a thin plate, 3 is a stand, 4 is a supply pipe, and 6 is a discharge pipe.

Front page continuation (72) Yoshihiro Oishi, Yoshihiro Oishi, Nagakute-cho, Aichi-gun, Aichi 41, No. 1 Yokomichi, Yokosuka Central Research Institute Co., Ltd. Address 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Kazuyuki Nakanishi 41 Aichi Nagakute Town, Aichi-gun, Nagakage Yokodoko 41 Address 1 Toyota Central Research Institute Co., Ltd.

Claims (3)

[Claims] 1. A thin sheet made of maraging steel is charged into a nitriding furnace, the residence time is 600 to 3600 seconds, and the decomposition rate is 1 to 6.
%, Gas nitriding is performed for a treatment time of 1 to 6 hours in an ammonia gas atmosphere of 10%, and a method for gas nitriding a thin sheet of maraging steel. 2. The method for gas nitriding a thin sheet of maraging steel according to claim 1, wherein the treatment temperature is 420 to 470 ° C. 3. The method for gas nitriding a maraging steel thin plate according to claim 1, wherein the maraging steel thin plate is a hoop having a CVT steel belt having a wall thickness of 0.10 to 0.3 mm.